1. Field of the Invention
The present invention relates to an integrated circuit, and more particularly to an integrated circuit operated for reducing a popping sound.
2. Description of the Related Art
Audio amplifier can be categorized as analog and digital classes. Analog audio output apparatus is controlled by an analog audio source, while a digital audio output apparatus is controlled by a pulse width modulated audio source. A digital audio output apparatus generally includes, as shown in FIG. 1, a high-side FET M1, a low-side FET M2, an LC low-pass filter 10, a capacitor C2, and a speaker 12.
The high-side FET M1 and the low-side FET M2 are coupled in series between a positive power rail VCC and GROUND. The high-side FET M1 and the low-side FET M2 are controlled by a high-side PWM signal (PWMH) and a low-side PWM signal (PWML), respectively. Both PWMH and PWML signals have substantially the same pulse width but are opposite to each other in phase. The LC low-pass filter 10 consisting of inductor L1 and capacitor C1 is used to filter out the PWM switching noise, and the capacitor C2 is used to filter out a direct current component.
It has been observed that a popping sound is often produced at the audio output of the speaker 12 during startup regardless of analog or digital audio output amplifier. The popping sound is offensive to the ears and may damage the speaker 12. FIG. 2 shows a block diagram of an electronic device having a sound output module disclosed in U.S. Pat. No. 7,046,082. Referring to FIG. 2, when an ON-OFF control signal Vpon has increased from a low level to a high level, a bias voltage Vb2 gradually increases according to a time constant determined by resistance values of resistors 21, 22 and a capacitance value of a capacitor 23. Since an output signal Vout of an output amplifier 26 increases in the same manner as does the bias voltage Vb2, a voltage Vx of a point X and a voltage Vy of a point Y also gradually increase. Consequently, since the voltage Vy changes gradually, the popping sound generated by a speaker Sp can be limited to a low level.
While the prior art solution to the problem of startup transients may be effective for analog applications, the circuit is only implemented in a pure analog circuit, and signals are transmitted in analog form. It is therefore desirable to provide a circuit and a method for generating PWM signals to reduce the popping sound at power-up or power-down for the digital audio amplifier as well.